The use of induction welding in the manufacture of steel tubing using a continuous roll-forming mill has become well accepted due to the many advantages of induction welding over electrical resistance welding or gas welding. Of the more important advantages are lower overall cost, increased production rates, reduced mill downtime for welding unit part replacement or repair and no marks on the outside surface of the formed tubing.
The induction welding process uses an induction coil positioned generally coaxially around the incipient tubing adjacent to where the lateral edges of the strip are brought into abutment. A high frequency current (200-600 kHz) is applied to the induction coil which induces current in the incipient tubing in a path including the location where the lateral edges first abut (which has relatively high resistance compared to that of the remainder of the path). This causes the lateral edges to become plasticized, and their passage between weld rollers completes the welding process.
In essence, the induction coil acts as the primary of a high frequency transformer, and the tubing, with its open seam, acts as a single turn secondary. There is a tendency of radio frequency currents to flow near the surface of the conductor. Two possible current paths in the tubing are of interest. The first path includes the outside surface of the tubing and the location where the lateral edges abut. Current taking this path results in heating of the lateral edges so that welding can take place. In the second path, current returns to the outside surface by flowing around the inside tube surface. This current flow does not appreciably heat the lateral edges. To raise welding efficiency, current flow in the first path is increased while current flow in the second path is decreased.
One way to achieve this is to insert an impeder inside the induction coil adjacent the inner tubing surface. The impeder contains ferromagnetic core material having high permeability. Its use increases the inductance and therefore the impedance of the circuit of the second path, thereby reducing the current in the second path. The cores of prior art impeders are formed by one or more solid rods of a material, such as ferrite, which, while having ferromagnetic properties, is a poor conductor of electricity to reduce eddy-current losses. While such rods are typically cooled, by passage of fluid along them, to maintain them below their Curie temperature so they retain their magnetic properties, they experience localized heating due to hysteresis losses. This can result in breakage of the rods forming the core and making core replacement necessary before efficient welding can continue. For additional information about the structure and operation of prior art impeders, reference may be made to U.S. Pat. No. 4,314,125.